To a great extent, the availability of water determines the economic value and productivity of land. Indeed, in some water-deficient western counties, the water rights that allow the holder to irrigate the land are worth more than the land. Even the most casual naturalist can easily observe the generality that where rainfall is abundant, plant life is abundant. This lecture touches on the subject of water management as it relates to soil.
1. Soil water balance sheet. Water enters and exits the surface layers of soil. One can keep a balance sheet to tally up incoming and outgoing water, thus quantifying or predicting the amount of water available in the soil.
Water In includes:
precipitation irrigation run-on
Water out includes:
run-off percolation evaporation transpiration
Adding all water in, then subtracting all water out, yields net soil water. One can then attempt to optimize these factors to improve amounts of soil water. Some factors are more manageable than others. Irrigation is the most direct and powerful tool. Most of the U.S. water supply is used for irrigation.
2. Water resources. Many countries have insufficient water supplies. Some examples follow.
U.S. has 2.6 million gal of accessible water per person Mexico has 1.0 million gal/person Jordan has 42,000 gal/person Israel has 98,000 gal/person ½ million gal/person is a critical threshold
Fresh water sources include polar ice caps, surface water (lakes, rivers), and ground water (much more plentiful than surface water).
3. Water depletion. Many important sources of water are becoming severely depleted. The Ogallala aquifer is about 24% depleted. Irrigated acreage above the Ogallala has declined 34% since 1974. The Gulf Coast has 60% overdraft (60% more out than in). The Mexico City aquifer has poor quality water and land above the aquifer has subsided about 7 meters. The water level drops 1 meter/year. Nearly an identical story is unfolding in Beijing. Ownership of many river waters is disputed. Ethiopia plans to divert Nile water. Syria diverts Euphrates river water. In the western U.S., water rights are presently being sold to cities, and well permits are controlled.
4. Irrigation is still important. Although only 16% of world's cropland irrigated, irrigated acres produce 30% of our food. See Tables 16-2 and 16-3 in the textbook for specific irrigation data.
5. Water quality for agriculture. Water quality for irrigation is affected by some minor factors such as BOD, turbidity and temperature. The more important factors are salinity, sodicity, and toxic substances.
Salinity in water can lead to saline soil. Salinity is measured by electrical conductivity (EC) in the units of decisiemen/meter (dS/m). A useful relation is: dS/m x 640 » ppm total soluble solids. Plants differ in salt tolerance. Normally one does not want over 2 dS/m for irrigation. Irrigation usually results in salt build-up in the soil. This problem is managed by applying a leaching fraction (see Figure 16-6 in the textbook).
Sodicity in water can create a sodic soil that is impermeable to subsequent additions of irrigation or rainwater. Sodicity is evaluated by sodium adsorption ratio (SAR). SAR values over 9 can indicate a problem. SAR is often adjusted for bicarbonate because bicarbonate precipitates calcium from the soil-water system:
OH- + HCO3- + Ca2+ ® CaCO3 + H2O
Specific ions and other materials can be toxic to plants or to the organism that consumes the plants. Cl, B, and Na are the most likely culprits. The toxicity of these elements depends on the irrigation method used. See Table 16-6 in the textbook for water quality standards.
6. Methods of Irrigation. Furrow irrigation is an old method used by native Americans before white settlers inhabited the land. This method is still used for small fields, and high-value crops. Sprinkler irrigation made major gains in acreage during the third quarter of this century. Sprinklers can be configured to pivot about a central point thereby irrigating a large circle such as a quarter-section, can be lateral travelling systems, or can be stationary. The present trend in sprinklers is towards Low Energy Precision Applicators (LEPA) which direct the water more precisely to the plant in need, rather than throwing water into the air where much of it evaporates. Microirrigation, long popular in desert lands such as Israel, is gaining popularity in the United States. Microirrigation includes various forms of drip and trickle emitters that deliver water under highly controlled conditions. This method is conducive to high water-use efficiency. Each irrigation method has unique management requirements.
Students are encouraged to look up the following vocabulary words in the textbook glossary or elsewhere and to browse the following web sites.
Sustainable agriculture web page with lots of links for water resources.
A web page from Germany with a comprehensive library of irrigation
A web page from a well known company (no endorsement implied) supplying
irrigation technology and hardware.
The web page for the American Water Resources Association.
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